The present invention relates to a composition based on a polyvinyl chloride resin, which is referred to as a PVC resin hereinbelow, having improved thermal stability and a method for the preparation thereof. More particularly, the present invention provides a PVC resin composition useful as a molding material imparted with remarkably improved thermal stability even when the heat stabilizer compounded therein has relatively low effectiveness such as those having no toxicity against the human body.
When a PVC resin-based composition is used as a molding material of various articles used in contact with foods or used in medical applications, it is essential that the heat stabilizer compounded therein has no toxicity against the human body such as the calcium-zinc based ones and barium-zinc based ones. The effectiveness of these non-toxic heat stabilizers is, however, rather low so that the shaped articles of the molding composition compounded with such a non-toxic heat stabilizer sometimes cause troubles in respect of the initial coloring after molding and thermal degradation during use. As a measure to solve this problem, it is practiced that the PVC resin-based composition is further admixed with an antioxidant or a compound having one or more hydroxy groups such as glycerin, pentaerithritol, sorbitan fatty acid esters, sugar esters and the like. These prior art methods are not quite effective in respect of the improvement of thermal stability or, rather, the addition of such additives may cause an adverse influence on the thermal stability of PVC resin-based compositions to increase the initial coloration when the heat stabilizer compounded therein is a tin-based or lead-based one.